Aggregation and surface properties of F-specific RNA phages: implication for membrane filtration processes

We report an experimental investigation of the electrokinetic properties and size variations of four F-specific bacteriophages of the types MS2, GA, Qbeta and SP (21-30 nm in diameter) over a broad range of pH values (1.5-7.5) and NaNO3 electrolyte concentrations (1-100 mM). The results obtained by dynamic light scattering show that the aggregation of SP and GA particles takes place over the whole range of pH and ionic strength conditions examined. For MS2 phages, the aggregation of MS2 particles is not observed for pH higher than the isoelectric point (pI) and large ionic strengths for which interparticular repulsive electrostatic interactions are however expected to be sufficiently screened. Aggregation of the MS2 phages, dispersed in 1 and 100 mM electrolyte concentration, occurs at pH 4, which basically corresponds to the pI as determined by electrophoresis measurements. The Qbeta particles suspended in solutions of low electrolyte concentrations aggregate at low pH values (pI approximately 3) and, unlike MS2, at large ionic strengths over the whole range of pH conditions considered in this study. These elements allow the determination of the hydrophobic sequence for the four phages SP approximately GA>Qbeta>MS2. Close inspection of the electrokinetic results reveals small to significant variations of the pI values-depending on the phage considered-with respect to the concentration of indifferent NaNO3 electrolyte. This indicates that features other than chemical and electrostatic in nature play a key role in determining the pI and more generally the electrophoretic mobility mu of viral particles. A qualitative interpretation is given and is based on the consideration of inner electro-osmotic flow within the isolated or aggregated particles. The impact of the flow properties within the particles is further in agreement with recent theoretical formalism developed for the electrokinetics of soft multiplayer particles, the phages analyzed here being some illustrative examples. The determination and qualitative interpretation of the surface properties of the viral particles as reported in the current study are commented within the context of water treatment especially concerning viral removal by membrane filtration processes.